What is NIST Calibration?

NIST stands for the National Institute for Standards and Technology. This organization is responsible for maintaining the master for different measurements, such as the standard "foot", standard, "second", or standard "pound". NIST also maintains the standard for temperature, humidity and pressure.

NIST Traceable means that the measuring standard can be traced directly back to the master standard maintained by the NIST organization.

NIST-Traceable does not tell you the measurement accuracy of a metrologty service in any detail. Only metrology services that can document their ISO 17025 standard provided data on their best measurement uncertainty.

Is the Chart Recorder sitting on or mounted to a surface prone to vibration?

For Temperature units try removing the unit from the application and placing it in an isolated, low air flow environment like a zip lock freezer bag or desk drawer. If the pen traces straighten out, the traces may have actually been caused by the changes in your application.